1. Field of the Invention
Systems and methods consistent with the present invention determine mass air charge for internal combustion engines, and more particularly to determine mass air charge for internal combustion engines which have separate intake runners and throttle valves operating with each engine cylinder or group of engine cylinders.
2. Description of the Related Art
The need for increased control of internal combustion (IC) Engines has been an ever-present requirement. Over the years, control systems targeted to IC engines have become more sophisticated and complex in order to meet the needs of stricter environmental and operational constraints. One focus of IC engine control system development relates to maintaining an accurate air-fuel ratio (AFR) over all engine conditions. Determination of AFR requires accurate determination of the amount of air mass entering the engine during intake stroke. The amount of air mass is then matched with an appropriate fuel mass to achieve proper engine operation. Methods of controlling the metered fuel accurately with the use of fuel injectors having predetermined flow rates are known. However, determining air mass is more difficult.
One method of determining the mass of air is by using of a mass air flow sensor (MAF) in order to directly measure the mass of air during the intake air cycle. Mass air sensors are accurate during steady-state operation. However, during transient events, for example engine acceleration, throttle position change, etc, errors due to manifold volume effects (also known as manifold filling/emptying) result from the use of mass air sensors.
To determine the mass of air actually entering an engine intake port, a known speed density (SD) method is used. The SD method uses the ideal gas law to calculate the density of the air in a cylinder in an indirect manner from measured pressure in the intake manifold. From this measured intake manifold pressure, the cylinder volume and volumetric efficiency (VE), a determination of actual mass air charge in the cylinder can be obtained. The SD method can be used by itself or combined with a MAF sensor for increased accuracy over all engine operating conditions.
Cylinder pressure is determined using a manifold absolute pressure (MAP) sensor to sample the pressure of the intake manifold. At the bottom dead center (BDC) position of a piston in a cylinder bore, and the output signal of the MAP sensor is sampled, and this sampled value is used to determine air pressure in the cylinder, and hence, cylinder air density. For engine configurations having a common intake manifold plenum, i.e., all of the cylinders open to a common intake manifold, pressure may be determined for each cylinder using a single MAP sensor by sampling the MAP sensor output at the BDC position of each cylinder.
However, there exists a class of IC engines which uses individual throttles and individual intake runners for each cylinder. This arrangement is known as an individual throttle body (ITB) arrangement. This arrangement does not have a common intake plenum area. Instead, each intake throttle and runner is isolated from the others. Since there is no common intake plenum on ITB arrangements, sampling manifold absolute pressure is more difficult.
In one prior art method of sampling manifold absolute pressure, small diameter flexible tubing is used to connect each runner into a single point (using multiple “T” couplings) where the MAP reading is sampled. However, the small diameter of tubing can degrade the absolute pressure reading. In addition, while one cylinder is undergoing an intake stroke the pressure reading on the common-tubing is being altered by the other intake throttle plates. Thus, the overall pressure is not an accurate representation of the cylinder under intake stroke, and is instead closer to an average of each intake runner with delay components introduced due to flow delays in the sensor tubing.
Existing aftermarket ITB engine control units (ECUs) utilize a single MAP sensor and, correspondingly, a single MAP signal detection input. Generally, the signal generated by a MAP sensor is an analog voltage which is proportional to absolute pressure. On the ECU, a single channel Analog-to-Digital Converter (ADC) is used to sample the analog voltage signal generated by the MAP sensor. ADC channels are precious resources on microprocessor-based ECUs, and there are many analog voltage signals from various sensors that require input channels to the ECU. Using separate MAP sensors on engines having a high number of cylinders is not practical with the limited number of on-board ADC channels.
FIG. 1 is an illustration of an IC engine depicting a conventional intake tract and cylinder for a single ITB. Referring to FIG. 1, a single engine cylinder arrangement 10 utilizing an ITB throttle body 16 and intake runner 12 is illustrated. Air flow into a cylinder 17 is controlled by ITB throttle plates 11 and is admitted into the intake runner 12. Engine air intake is controlled by intake valve 13. Piston 14 moves within the single engine cylinder arrangement 10.
It should be noted that a multi-cylinder ITB arrangement has multiple cylinders of the arrangement illustrated in FIG. 1, each operating independently from one another. Pressure in each intake runner is independent as well. The pistons are arranged at different positions within each cylinder, phased with respect to crankshaft rotation according to the number of cylinders. For example, in a 4-stroke cycle engine, the phasing between cylinders is about 720 degrees of crankshaft rotation divided by the number of cylinders. For a 2-stroke cycle arrangement phasing is generally about 360 degrees of crankshaft rotation divided by the number of cylinders. Odd fire engine geometries will slightly alter the relative phase between cylinders. However, the concept is still the same. It should be noted that only one cylinder is undergoing an intake event at any given time. It is the cylinder undergoing an intake event for which the intake mass air calculation is desired, so that an appropriate fuel/air mixture can be determined by the time the compression stroke is started and fuel needs to be injected.
In order to determine the cylinder pressure for the speed-density control calculation, an intake runner pressure port 15 is utilized. In an exemplary embodiment of the present invention, a sampling event is correlated with piston position at BDC of an intake stroke. This is the point where the cylinder volume is at maximum, and correlates approximately to minimum cylinder pressure.
FIG. 2 is an illustration of a prior art cylinder pressure sampling arrangement utilizing a single MAP sensor interfaced with multiple cylinders utilizing ITB manifolds. In FIG. 2, only two cylinders 10 are shown, however the illustration may be extended to any number of cylinders. This configuration sums all of the intake runner pressures utilizing a pressure channel interconnect setup.
Referring to FIG. 2, tubes 20 and 21 made of rubber, plastic, or metal, or casting channels, come together at a summing point 22. The pressure at the summing point 22 is measured by a standard MAP sensor arrangement 23. The pressure at the summing point 22 is converted to a voltage signal 24 and input to the ECU 25. At any instant in time the pressure signal at the summing point 22 represents the approximate sum of all of the intake runner pressures. Since each cylinder is at a different operating point of the Otto cycle, the pressure at the summing point 22 is proportional to the sum of the absolute pressures in each intake runner 12.